1. Technical Field
An embodiment of the invention relates generally to integrated circuit manufacturing, and in particular relates to pellicles used in lithographic exposure operations.
2. Description of the Related Art
Reticles and photomasks are used during the manufacture of integrated circuits. Pellicles are used to protect the reticles and photomasks, keeping dust particles and other contaminants off the reticle/photomask surface, where the particles would otherwise cause defects in the exposure image. A pellicle is placed a predetermined distance from the reticle/photomask, the distance calculated in such a way that the dust particle image is not projected on the wafer. The exposure radiation must by necessity pass through the pellicle, so the pellicle needs to be essentially transparent at the wavelengths being used. Pellicles used with ultraviolet radiation are commonly made of a thin polymer film that is transparent at the required wavelengths.
As the features of integrated circuits become smaller, shorter wavelengths of exposure radiation are required. However, in the vacuum ultraviolet (VUV) ranges of less than 193 nanometers (nm), the radiation can cause the polymer material in conventional pellicles to deteriorate by generating free radicals. The free radicals not only destroy the physical and optical qualities of the polymer, but also generate reaction products that absorb the radiation and generate more free radicals. Thus the transmissiveness of the polymer at the desired wavelength degrades at an accelerating rate, greatly shortening the effective life cycle of the pellicle, and thereby limiting the number of exposure operations the pellicle/reticle assembly can be used for before it must be replaced.